This invention relates to a satellite communication method and network and, more particularly, to synchronization of clocks used in terrestrial or earth stations of the network for use in measuring a propagation delay time between any two of the terrestrial stations.
As a method of measuring a propagation delay time between two terrestrial stations, it is a most basic method to determined the propagation delay time by making each station transmit data to a satellite of the network and receive the data therefrom, calculating a one-way propagation delay between it and the satellite, and calculating a sum of the one-way propagation delays calculated for the two stations between which communication should be carried out.
In order to measure the propagation delay time in connection with each datum, it is necessary to make all terrestrial stations have a common clock. Each of the terrestrial stations comprises a local clock indicative of a current time which advances periodically. A reference station of the network comprises a reference clock indicative of a reference time. In order to make the local clocks of the terrestrial stations synchronously indicate a common time, it is known to make the reference station simultaneously announce the reference time to the terrestrial stations through a communication link established by the reference station to the terrestrial stations through a satellite of the network. In each terrestrial station, its current time is set with reference to the reference time received through the communication link.
The reference station may be a center station which is operable also as one of the terrestrial stations. Including the center station, each terrestrial station can establish a bidirectional link to and from at least one selected station of the terrestrial stations and transmit a transmission signal to the selected station or receive a reception signal from the selected station through the bidirectional link. It is known to use a packet in each of the transmission and the reception signals.
In a manner which will later be described, each terrestrial station of a conventional satellite communication network comprises a local clock indicative of the current time. A packet forming arrangement forms the transmission packet to which a local time datum is attached to indicate the current time of production of the transmission packet. A data extracting arrangement extracts a particular time datum from the reception packet received from the selected station. A calculation unit calculates the propagation delay time. A first station of the terrestrial stations transmits a first packet as the transmission packet. Similarly, a second station of the terrestrial stations transmits a second packet as the transmission packet to a particular link established by the second station as the bidirectional link to and from the first station. The second packet carries the particular time datum, which indicates the current time in the second station when the second packet is formed. The first station receives the second packet as the reception packet.
By the basic method of determining the propagation delay time from the one-way propagation delays at two stations, it is impossible to measure fluctuations in the propagation delays between data due to a variation in traffic, performance of a link access system, and a processing time in each terrestrial station.
It is possible in the manner described above to real-time calculate a propagation delay time per datum by using a conventional time synchronization method to time synchronize all terrestrial stations, by making each station use a current time thereof when transmitting data, and by calculating a difference from a current time of a receiving station. This, however, gives rise to a problem such that it is impossible to correctly measure the propagation delay time due to an error introduced thereinto by a geographic difference between the terrestrial stations as will be described later.